1. Field of the Invention
The present invention relates to an optical variable attenuator suitable for use as an equalizer that compensates for the wavelength dependence of the gain of a repeater constituting an optical communication system such as an optical submarine cable or of the loss of an optical signal in the optical cable.
2. Description of the Background Art
With the view to increasing its transmission capacity, an optical submarine cable system or similar optical communication system usually employs a WDM (Wavelength Division Multiplexing) transmission scheme that multiplexes optical signals of plural wavelengths. To enhance the characteristic of such WDM transmission, it is necessary that the optical signal level of each wavelength of the wavelength-multiplexed signal be held as constant, as possible. To meet this requirement, an equalizer is inserted in the transmission line that compensates for the gain-wavelength characteristic of a repeater or the wavelength loss characteristic of the optical signal by the transmission line. More specifically, the optical signal level of each wavelength of the wavelength-multiplexed signal is held constant by using an equalizer that has a transmission loss characteristic opposite to the sum of wavelength characteristics of the repeater and the transmission line.
However, when the gain-loss wavelength characteristic of the repeater or optical cable changes due to its deterioration during operation of the optical submarine cable system, the transmission loss characteristic of the equalizer does not correspond with the sum of wavelength characteristics of the repeater and the transmission line, resulting in a failure to hold the optical signal of each wavelength of the wavelength-multiplexed signal at a constant level. This incurs the possibility of decreasing the total amount of transmission of the optical submarine cable system.
Such a problem could be solved by using an equalizer whose transmission-loss characteristic is variable even during operation of the optical submarine cable system. That is, when the repeater or optical cable deteriorates, the optical signal of each wavelength of the wavelength-multiplexed signal can be held at a constant level by suitably adjusting the transmission loss characteristic of the equalizer to correspond with an inverse characteristic of the sum of the wavelength characteristics that the repeater and the transmission line have after its deterioration.
FIG. 1 is a diagram depicting the construction of a conventional optical variable attenuator that is used as an equalizer whose transmission loss characteristic is variable as described above. In FIG. 1, reference numeral 1 denotes generally a prior-art optical variable attenuator, which controls the intensity of the optical signal input thereto and outputs an optical signal of a desired intensity. Reference numeral 2 denotes a polarizer placed at the input side of a Faraday rotator 4, which extracts a predetermined polarized wave from the input optical signal and outputs the extracted wave. Reference numeral 3 denotes a polarizer placed at the output side of the Faraday rotator 4, which outputs only a predetermined polarized wave component of the optical signal having its plane of polarization rotated by the Faraday rotator 4. Reference numeral 4 denotes a Faraday rotator formed of a paramagnetic material, which rotates the plane of polarization of the optical signal from the polarizer 2 in accordance with the intensity of the magnetic field produced by an electromagnetic coil 5. Reference numeral 5 denotes an electromagnetic coil that provides a magnetic field to the Faraday rotator 4. Reference numeral 6 denotes a feeding terminal of the electromagnetic coil 5, through which a control current from a control current supply circuit 7 is supplied to the electromagnetic coil 5. Reference numeral 7 denotes a control current supply circuit, which is capable of suitably changing the current value of the control current supply to the electromagnetic coil 5 in accordance with a change in the gain-wavelength characteristic of the repeater or in the loss-wavelength characteristic of the transmission line in the optical communication system.
Next, the operation of the prior art example will be described below.
In the first place, an optical signal sent in the direction of the arrow in FIG. 1 is rendered into parallel rays by a lens (not shown) placed at the input side of the optical variable attenuator 1, thereafter being input to the polarizer 2. The polarizer 2 extracts a predetermined polarized wave from the above-mentioned optical signal, and inputs it to the Faraday rotator 4. To the Faraday rotator 4 is being applied from the electromagnetic coil 5 a magnetic field of an intensity corresponding to the value of the control current fed thereto. The Faraday rotator 4 rotates the polarized wave of the optical signal through an angle corresponding to the intensity of the above-mentioned magnetic field. Following this, only a predetermined polarized wave component is extracted by the polarizer 3 from the optical signal fed thereto from the Faraday rotator 4 and is output from the optical variable attenuator l.
With the control current to the electromagnetic coil 5 set at a suitable value by the control current supply circuit 7, it is possible to control the intensity of the magnetic field to be applied to the Faraday rotator 4. This permits selective setting of a desired polarization-rotating angle for the Faraday rotator 4. Accordingly, the optical signal to be emitted from the optical variable attenuator 1 is set at a desired intensity by selecting the polarized wave component of the optical signal that is extracted by the polarizer 3.
Thus, even if the gain-wavelength characteristic and loss-wavelength characteristic of the repeater and the optical cable vary due to their degradation, the variations can be compensated for during operation of the optical submarine cable system.
The concept of the above-described optical variable attenuator 1 is disclosed in such prior art literature as Japanese Patent Application Laid-Open Gazette No. 212315/95. The prior art literature concerns an optical isolator, not the optical variable attenuator, but it discloses a concept common to the optical variable attenuator 1 in the usage of an electromagnetic coil to make variable the intensity of the magnetic field to be applied to the Faraday rotator.
With the conventional optical variable attenuator of the above construction, since the Faraday rotator 4 is formed of a paramagnetic material, a continuous supply of a fixed-value current to the electromagnetic coil 5 is needed to maintain the polarization-rotating angle constantxe2x80x94this gives rise to a problem that the supply of the above-mentioned fixed-value current may sometimes become impossible due to secular changes of such a current path as the electromagnetic coil 5 or feeding terminal 6 and such a current supply source as the control current supply circuit 7.
That is, the conventional optical variable attenuator is not suitable for use in an optical communication system such as an optical submarine cable system that is difficult of frequent maintenance of parts used therein but requires long-term reliability.
A possible solution to the above-mentioned problem is to form the Faraday rotator by a ferromagnetic substance. The reason for this is that the Faraday rotator formed of the ferromagnetic substance maintains the polarization-rotating angle by its spontaneous magnetization even if the power feed to the electromagnetic coil is stopped.
Since the spontaneous magnetization develops stably in only two opposite directions, however, the polarization-rotating angle of the Faraday rotator can be set at only two values. Accordingly, the transmission loss characteristic of the optical variable attenuator can also be set at only two valuesxe2x80x94this makes it impossible to flexibly deal with variations in the gain-loss wavelength characteristic by the deterioration of the repeater or optical cable.
The present invention is intended to solve the above-mentioned problems, and has for its object to provide an optical variable attenuator that keep the transmission loss characteristic unchanged even if the power feed to the electromagnetic coil is stopped and permits fine setting of the transmission loss characteristic.
An optical variable attenuator according to an aspect of the present invention comprises: a plurality of polarizer means for extracting a predetermined polarized wave component from an input optical signal; polarized-wave rotating means composed of an array of Faraday rotators formed of a ferromagnetic material and arranged in series between the plurality of polarizer means, and magnetic field supply means provided for each Faraday rotator, for supplying thereto a magnetic field for rotating the polarized wave component of the optical signal; and polarization-rotating angle control means for setting polarization-rotating angles of the Faraday rotator array based on a combination of polarization-rotating angles each set for one of the Faraday rotators by controlling the intensity of the magnetic field supplied to each Faraday rotator from the magnetic field supply means.
With the above construction, since the Faraday rotators are each formed of the ferromagnetic material, it is possible to maintain the transmission loss characteristic of the optical variable attenuator by the spontaneous magnetization of the Faraday rotators without the need for continuously applying thereto magnetic fields from the outside and to finely set the transmission loss characteristic; hence, the optical variable attenuator of the present invention is suitable for use in an optical submarine cable system or similar optical communication system that is difficult of frequent maintenance of the parts used therein and requires long-term reliability.
In an optical variable attenuator according to another aspect of the present invention, the Faraday rotator array comprises at least two or more Faraday rotators of different polarization-rotating angles, and the polarization-rotating angle control means sets the polarization-rotating angles of the Faraday rotator array based on a combination of the different polarization-rotating angles each set for one of the Faraday rotators.
The above construction permits fine setting of the transmission loss characteristic of the optical variable attenuator.